The myofilaments of cardiac and skeletal cells are critically dependent on the intracellular concentration of Ca2+ for regulation of their contractile activity, In turn, the intracellular concentration of Ca2+ is controlled by membrane structures which are endowed with specific transport systems for the divalent cation. It is the aim of this proposal to combine ultrastructural, biochemical and physiological methods of investigation on order to clarify the mechanisms of Ca2+ uptake and release by these membranes. The proposed research will be carried out with a range of experimental systems including dissociated cells, isolated sarcoplasmic reticulum vesicles, solubilized and purified proteins (Ca2+ ATPase) and reconstituted membranes. Specific ultrastructural and/or conformational features of these systems will be related to functional activities with specific reference to active Ca2+ transport and coupled enzyme activity. Sequential steps of the enzyme cycle will be studied by rapid mixing techniques in the forward and reverse directions, leading to active transport and ATP synthesis respectively. Passive Ca2+ fluxes will be also studied, and their relevance to contractile regulation will be evaluated by monitoring sarcomere activation in dissociated cell preparations. Comparative studies will be performed under the influence of pharmacological agents. These studies will lead to clarification of basic biological mechanisms: ion transport, energy transduction and contractile control.